The present invention relates to welded wire fabric for use particularly in making cylindrical reinforcing cages for reinforcing concrete pipe. It is particularly advantageous for use in conjunction with packer head type pipe manufacturing machines and similar machines which impart a torsional force to a reinforcing cage as concrete is formed around it.
Packer head type pipe making machines are the most commonly used pipe manufacturing equipment for small diameter concrete pipe. Basically, the manufacturer first inserts a cylindrical reinforcing cage into the cylindrical pipe form. The roller head is then inserted all the way to the bottom of the form. The head is then activated to simultaneously inject concrete into the form, rotate and move gradually upwardly towards the top of the form. The rotating head is gradually moved upwardly as it is rotated and as concrete is cast around the reinforcing cage until the pipe has been completed and the rotating head is drawn out of the top of the form.
One quality control problem which is encountered in manufacturing pipe in this manner is that once the bottom of the cage is embedded in concrete, there is a tendency for the cage to twist as the packer head continues to rotate and move upwardly. The rotating packer head and the movement of concrete applies a substantial torsional force on the cage. The problems created are twofold. First, the twisting tends to bring the cage too close to the inside wall of the pipe or possibly at points too close to the outside wall of the pipe if the cage gets skewed by the twisting action. Sometimes, fabric actually sticks through the pipe wall, in which case the pipe has to be rejected at the outset.
A second problem which is somewhat more insidious in nature is that the twisting of the cage sets up built in stresses within the wall of the pipe, even if the cage does not actually project through the inside or outside wall surface. In other words, even after the pipe is formed, the cage inside is trying to twist back to its original shape, thereby placing stresses on the concrete walls which can lead to cracking or comparable decay of the pipe before or during curing.
Heretofore, the first problem discussed above is often blamed on improper formation of the cage, rather than on torsional twisting of the cage during manufacture of the pipe. Artisans have attempted to solve the second and more insidious problem by either making two passes with the packer head, one clockwise and the other counterclockwise, or by placing centering arms on the packer head shaft housing above the roller head. The ends of these arms engage the cage and slide up it as the roller head moves up.